The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
During combustion in a diesel engine, an air/fuel mixture is delivered through an intake valve to cylinders and is compressed and combusted therein. After combustion, the pistons force the exhaust gas in the cylinders into an exhaust system. The exhaust gas may contain oxides of nitrogen (NOx) and carbon monoxide (CO).
Exhaust gas treatment systems may employ catalysts in one or more components configured for accomplishing an after-treatment process such as reducing nitrogen oxides (NOx) to produce more tolerable exhaust constituents of nitrogen (N2) and water (H2O). Reductant may be added to the exhaust gas upstream from an after-treatment component, such as a selective catalyst reduction (SCR) component, and, for example only, the reductant may include anhydrous ammonia (NH3), aqueous ammonia or urea, any or all of which may be injected as a fine mist into the exhaust gas. When the ammonia, mixed with exhaust gases, reaches the after-treatment component, the NOx emissions are broken down. A Diesel Particulate Filter (DPF) may then capture soot, and that soot may be periodically incinerated during regeneration cycles. Water vapor, nitrogen and reduced emissions exit the exhaust system.
To maintain efficient NOx reduction in the after-treatment component, a control may be employed so as to maintain a desired quantity of the reductant (i.e., reductant load) in the after-treatment component. As exhaust gas containing NOx passes through the after-treatment component, the reductant is consumed, and the load is depleted. A model may be employed by the control to track and/or predict how much reductant is loaded in the after-treatment component and to inject additional reductant as required so as to maintain an appropriate reductant load for achieving a desired effect such as reduction of NOx in the exhaust stream.
Service regeneration of the DPF is often conducted at elevated exhaust temperatures. Because of these increased temperatures, a flow of reductant through the injector(s) may be maintained so as to prevent thermal damage of the injector. Unfortunately, it can be difficult to predict how much of the reductant injected for such purposes is oxidized or otherwise consumed in the after-treatment component and how much may have survived and accumulated so as to contribute to the loading of the after-treatment component.
As a consequence, model estimates of ammonia load may be inaccurate, and may thus be rendered unreliable. In particular, experience has shown that following the occurrence of certain events, such as a DPF service regeneration event, load estimates based on models may deviate substantially from observed levels of NH3 load on the after-treatment component. Hence, diagnostic processes based on measurement and evaluation of NOx reduction efficiencies in the after-treatment component may produce erroneous results such as where more reductant is actually loaded on the after-treatment component than the diagnostic system assumes based on the inaccuracies in the model. In such situations, NH3 “slip” can occur wherein NH3 is interpreted by downstream sensors as NOx. Such errors are particularly problematic when using sensors that are cross-sensitive to both NOx and NH3. Similarly, where an actual NH3 load is substantially lower than the model estimate, the incorrect NH3 load can cause a worse than expected NOx reduction efficiency to be assessed by the diagnostic system, potentially resulting in an incorrect diagnosis and invocation of remedial measures to be taken.
Accordingly, it is desirable to provide a system and method for more accurately predicting a quantity of reductant (i.e., the reductant load) present on after-treatment components and for managing the operations through which NOx are reduced in such after-treatment components with improved reliability following one or more trigger events.